There are two arms of acquired immunity, which, while able to collaborate to achieve the common goal of eliminating antigen, are mediated by distinct participants of the immune system with different effects. One arm of acquired immune response, humoral immunity, is mediated primarily by B cells and circulating antibodies. The other arm, referred to as cellular or cell-mediated immunity, is mediated by T cells that synthesize and elaborate cytokines which affect other cells.
Activation and differentiation of B cells in response to most antigens requires that (1) B cells receive an antigen signal via their antigen specific receptor, membrane Ig, and (2) B cells receive contact dependent and independent signals from activated T cells. The contact dependent costimulatory signal results from ligation of the CD40 receptor on B cells to the CD40 ligand expressed on activated T helper cells. (Laman et al., Crit. Rev. Immunol., 16, pp. 59-108 (1996); Van Kooten and Banchereau, Adv. Immunol., 61, pp. 1-77 (1996)). Contact independent signaling is mediated by cytokines synthesized and elaborated by activated T cells. Together these contact dependent and independent signals drive B cells to differentiate to either (1) memory B cells poised to mediate a more rapid response upon secondary exposure to antigen, or (2) antibody secreting plasma cells. Plasma cells, which are the terminal differentiation stage of B cells, synthesize and secrete antibodies.
T helper cells (“Th”) play several significant roles in the immune system. Cytokines elaborated by Th cells at the onset of an immune challenge have been shown to affect which immune effector pathways are subsequently activated. Th cells are activated by the interaction of their antigen specific receptor with antigen-presenting cells (APCs) displaying on their surfaces peptide fragments of processed foreign antigen in association with MHC class II molecules. Activated Th cells, in turn, secrete cytokines (lymphokines) which activate the appropriate immune effector mechanisms.
Th cells can be divided into three subgroups Th0, Th1 and Th2, based upon their cytokine secretion patterns. (Fitch et al., Ann. Rev. Immunol., 11, pp. 29-48 (1993)). In mice, non-stimulated “naive” T helper cells produce IL-2. Short term stimulation of Th cells leads to Th0 precursor cells, which produce a wide range of cytokines including IFN-α, IL-2, IL-4, IL-5 and IL-10. Chronically-stimulated Th0 cells can differentiate into either Th1 or Th2 cell types, whereupon the cytokine expression pattern changes. Certain cytokines, for example IL-3, GM-CSF and TNF, are released by both Th1 and Th2 cells. Other cytokines are made exclusively by only one Th cell subgroup. (Romagnani et al., Ann. Rev. Immunol., 12, pp. 227-57 (1994)). Th1 cells produce LTα IL-2 and IFN-γ which activate macrophages and inflammatory responses associated with cellular immunity and resistance to intracellular infections.
Th2 cells produce the cytokines IL-4, IL-5, IL-6 and IL-10 which increase eosinophil and mast cell production and promote the full expansion and maturation of B cells. (Howard et al., “T cell-derived cytokines and their receptors”, Fundamental Immunology, 3d ed., Raven Press, New York (1993)). Th2 cells also participate in generating B cell memory, somatic mutation and thus affinity maturation, and in regulating de novo immunoglobulin isotype switching. For example, the Th2 cytokine IL-4 switches activated B cells to the IgG1 isotype while suppressing other isotypes. IL-4 also stimulates the overproduction of IgE in type I hypersensitivity reactions. The Th2 cytokine IL-5 induces the IgA isotype important in mucosal immunity.
The secondary lymphoid tissues, such as the lymph nodes (LN), spleen and mucosal lymphoid tissues, are highly efficient in trapping and concentrating foreign substances, and are the main sites of antigen driven activation and differentiation of T and B lymphocytes. These processes are dependent upon the diversity and organization of cells in these tissues, providing a framework for many aspects of humoral immune responses, such as T/B cell interactions, germinal center (GC) formation, affinity maturation, immunoglobulin class switching and cell trafficking. (Klein, J., Immunology, John Wiley and sons, (1982)). The molecular mechanisms responsible for the development, structural maintenance and function of peripheral lymphoid tissues are not fully understood.
Although the general structure of the secondary lymphoid tissues differs markedly and shows variations between species of mammalia, the fine structure of these secondary lymphoid tissues shares certain features, such as, for example: (1) antigen accessibility, (2) structural features ensuring continued contact of antigen with lymphocytes, (3) T cell rich areas surrounded by B cells, (4) B cell rich follicles, (5) marginal zone type sites, (6) specialized endothelial cells, and (7) antibody production sites, as discussed in further detail below.
The secondary lymphoid tissues are accessible to antigen in the system. For example, antigen accesses the spleen via the sinusoidal blood supply, the LN via the afferent lymphatic vessels, and is transported across specialized epithelium into the mucosal lymphoid tissue.
The secondary lymphoid tissues in various species also share certain structural features such as follicular dendritic cells (FDC) and interdigitating cells (IDC), which ensure the continued presence of antigen in the lymphocyte rich areas of the tissues.
Another common feature is the presence of T cell rich areas surrounded by B cells. T cell rich areas include, for example, the periarteriolar lymphoid sheaths in the white pulp of the spleen, and the paracortical region of LN, which contain large numbers of recirculating T cells and IDC, which in turn function as accessory cells for T and B cells.
Additionally, lymphoid tissues typically have B cell rich primary and secondary follicles in the white pulp of the spleen, and in the cortex of the LN. Secondary follicles in such lymphoid tissues are also called germinal centers (GC) and have a dense FDC network to capture and present antigens.
Marginal-zone type areas are also noted as defined histologic areas in the murine spleen and more diffuse sites in human secondary lymphoid organs. These areas are comprised primarily of marginal zone macrophages (MZM), metallophilic macrophages (MM), marginal zone B cells and reticular cells, but may also include T cells and dendritic cells. (Kraal, Int. Rev. Cytol. 132, pp. 31-74 (1992)). The opening of the arterial blood stream into the marginal zone areas gives antigens direct access to these cells and promotes cellular reactions to antigens at this site. (Kraal, Int. Rev. Cytol. 132, pp. 31-74 (1992)). The presence of MZM are also required for optimal trafficking of B cells in the splenic white pulp. (Kraal, 1992; Kraal, et al, Immunology, 68, pp. 227-232(1989)).
Typically, blood lymphocytes enter the secondary lymphoid tissues by crossing specialized endothelium, for example the endothelial lining of the venules of LN (high endothelial venules—HEV) and the endothelial lining of splenic blood sinusoids in the marginal zone-like structures. This endothelium expresses adhesion molecules and addressins which function in the trafficking of cells to secondary lymphoid tissues. For example, peripheral LN addressins (PNAd) are distinct from the mucosal LN addressin, MAdCAM-1, which is involved in trafficking of lymphocytes to mucosal lymphoid tissues, including tissues such as the mesenteric LN, Peyer's patches and lamina propria.
Not all addressins are clearly defined, for example, the addressin for lymphocyte homing to spleen remains undefined. The physiological roles of these addressins include enhancing recruitment of appropriate sets of antigen specific lymphocytes into an immune response, and subsequent dissemination of the immune response throughout the body.
Finally, the plasma cells, which are the antibody producing plasma cells, are detected at different locations from where the progenitor B cells are activated by antigen. For example, antibody produced by plasma cells in splenic red pulp mainly results from B cell activation in T cell zones, and plasma cells in the medulla of LN are derived from B cells activated in T cell zones of the same node. Similarly, antibody produced by plasma cells in bone marrow are derivatives of B cells activated in spleen and lymph node, and plasma cells in the lamina propria of gut mainly derive from B cells activated in mesenteric LN or gut associated lymphoid tissue.
See e.g., ICM MacLennan, “The Structure and Function of Secondary Lymphoid Tissues” in Clinical Aspects of Immunology 5th edition, eds. P. J. Lachman, Sir D. K. Peters, F. S. Rosen, M. J. Walport, Blackwell Scientific Publications pp 13-30 (1993).
In general, the cellular/histologic events underlying a humoral immune response to T dependent antigens are as follows (Toellner, et al., J. Exp. Med., 183, pp. 2303-2312 (1996)):
In the Inductive phase, naive B and T cells are activated and recruited into the immune response in the days immediately after antigen enters the body. In the spleen, for example, within 12 hours of immunization for a secondary response, memory B cells encounter blood-borne antigen in the marginal zone and leave the marginal zone to go to the T cell zones. B cells can be detected in the T cell zones within 24 hours. Immunoglobulin switch transcripts can be detected within 12 hours of secondary antigen exposure, thus indicating that the T-B cell interaction has already occurred. The B cells then migrate to the exit zones and red pulp where they proliferate to form foci of B cell blasts and differentiate into plasma cells. The B cells also continue to proliferate in the IDC rich T cell zone. Within 4 days after immunization, and after proliferation in the GC, B memory cell production will start. In a primary response, well developed GC are apparent by day 10 and reach peak size by day 14 post-immunization
T cell proliferation in the T cell zones becomes evident 48-72 hours and peaks on day 7 after immunization. This T cell proliferation contributes to T cell dependent B cell activation. Proliferative levels in the T cell zone decrease as GC forms. T cell proliferation also occurs in the GC where centrocytes (B cells) in the dark zone pick up antigen from IDC, and present antigen to T cells in light zone.
T cell dependent antigen can activate marginal zone B cells, newly produced naive B cells and recirculating lymphocytes attracted to and retained in secondary lymphoid organs by addressins and adhesion molecules. Naive B cells show the same kinetics for going to T cell zone, etc. as do the activated B cells.
Established Phase of T Cell Dependent Responses
The established phase of T cell dependent responses is maintained by the continued activation of memory B cells in the follicles of secondary lymphoid organs. There is very little recruiting of naive B cells at this stage, and the response is primarily driven by antigen retained on FDC. GC are required for optimal memory generation, isotype switching, somatic mutation and thus affinity maturation of immunoglobulin.
The mounting of such lymphocyte responses results in the production of antibodies able to circulate throughout the body by various routes, for example, antibodies leave the spleen via the blood, and exit LN via the efferent lymphatics. The antibodies thus encounter and directly bind to the invading pathogen. This recognition event sets off a cascade of immune effector mechanisms, including activation of the complement cascade and cellular reactions to mediate protection of the host from the pathogen.
Antibodies also play a role in some pathologic responses such as hypersensitivity responses—inappropriate or disproportionate immune responses evoked upon contact with a previously encountered antigen. There are four recognized types of hypersensitivity.
Type I “immediate hypersensitivity” involves allergen-induced Th2 cell activation and Th2 cytokine release. The Th2 cytokine IL-4 stimulates B cells to undergo isotype switching to produce IgE, which in turn activates mast cells to produce acute inflammatory reactions such as those which lead to eczema, asthma and rhinitis.
Types II and III hypersensitivity are caused by IgG and IgM antibodies directed against cell surface antigens or specific tissue antigens (Type II) or soluble serum antigens to form circulating immune complexes (Type III).
Type IV “delayed type” hypersensitivity (DTH) is a Th1 cell mediated response and can be transferred between mice by transferring Th1 cells, but not by transferring serum alone. This feature distinguishes Type IV DTH from the other three types of hypersensitivity, which require humoral immune responses caused primarily by antibodies which can be transferred in cell-free serum. (Roitt et al., Immunology, pp. 19.1-22.12 (Mosby-Year Book Europe Ltd., 3d ed. 1993))
Pathological humoral immune responses are associated with a number of organ-specific and systemic autoimmune conditions such as Systemic Lupus Erythematosus, Wegener's Granulomatosis, Polyarteritis Nodosa (PAN), Rapidly Progressive Crescentic Glomerulonephritis and Idiopathic Thrombocytopenia Purpura, as well as chronic inflammatory diseases such as the Graves' and Chagas' disease. Humoral immune responses may also contribute to grafted tissue and transplanted organ rejection.
The treatment of these various immunological conditions to date has generally employed immunomodulatory and immunosuppressive agents. Three general immunosuppressive agents currently used are steroids, cyclophosphamide and azathioprine.
Steroids are pleiotropic anti-inflammatory agents which suppress activated macrophages and inhibit the activity of antigen presenting cells in ways which reverse many pathologic T cell effects. Cyclophosphamide, an alkylating agent, mediates cell death by inhibiting DNA replication and repair. Azathioprine is an anti-proliferative agent which inhibits DNA synthesis. These non-specific immunosuppressive agents are generally required in high doses which increase their toxicity (e.g. nephro- and hepatotoxicity) and cause adverse side effects. They are thus unsuitable for long term therapies.
Thus, there is an unmet need for additional agents and therapies which overcome the problems caused by conventional treatments.